The Enduring Legacy of the Voyagers

byPaul GilsteronJuly 27, 2010

by Larry Klaes

The Faces from Earth project, run so energetically by Tibor Pacher, is planning its next ‘E.T. Are You Out There?’ campaign, following a successful campaign in May that introduced interstellar concepts to school children in five countries. In this piece, journalist Larry Klaes looks back at the Voyager spacecraft, which will be the subject of the new Faces from Earth campaign. The Voyagers electrified all of us with the discovery of volcanoes on Io and a possible ocean beneath Europa’s ice, and the ensuing stream of images from planets and moons never before seen up close. They also carried golden discs bearing information about their builders. As of this morning (EST), Voyager 1 is 15 hours, 44 minutes, 56 seconds in light-travel time from home, at the edge of the Solar System but, as Larry makes clear, hardly forgotten.

In the first decade of the Space Age, humanity succeeded in sending a handful of robotic space probes to Earth’s two nearest planetary neighbors, Venus and Mars. The voyages of these mechanical vessels, which only took a matter of months, were brief in their visits to these alien worlds. Nevertheless, these new kinds of explorers gave scientists their first knowledge of the true natures of these places after centuries of speculation.

Much farther beyond, where the Sun is eventually reduced in appearance to just a very bright star, is the realm of the outer gas giant worlds. These planets are many times larger than all of the inner terrestrial globes put together and lack solid surfaces in the same sense as our Earth and its celestial brethren. The Jovian planets also keep in their mighty gravitational grips collections of moons and rings of debris that would qualify them as whole solar systems in their own right.

But for humanity in the early days of space exploration, these alien places were very far away and full of unknowns, including whether a fast-moving spacecraft could navigate the natural boundary between the terrestrial and Jovian realms known popularly as the Asteroid Belt without being smashed to pieces by potentially deadly dust and meteoroids. In addition, a spacecraft of that era would take decades to reach all the outer worlds; such vessels were still on the proverbial drawing boards, while most of the actual probes which did reach the nearest worlds in functioning order often did so with a lot of luck and engineering skill.

Beginnings of a Grand Tour

In that same time, when the two main players of the Space Age were preparing to see who could place the first humans on the lunar surface, it was determined that the outer planets would align in such a way in their solar orbits in the late 1970s that they could be reconnoitered by a quartet of nuclear-powered space probes flying past each world in just one decade. The plan and the mission were appropriately named the Grand Tour.

Early on the project was threatened with termination, not by some hazard in space but by budgetary problems on Earth. To stay alive in NASA, the Grand Tour was scaled back to explore just the two nearest gas giants, Jupiter and Saturn. The vessel numbers were reduced from four to two: The remaining probes were christened Mariner 11 and 12, following in the line of American space probes that had opened the way to understanding the inner Solar System. By the time the vessels that remained from the initial outer worlds exploration plan were ready to be launched into the heavens in the late summer of 1977, there were a number of further significant changes to the mission.

Image: The Voyager spacecraft. Voyager’s ‘Golden Record’ can be seen attached to a panel of the spacecraft’s instrument housing. Credit: NASA.

Up front, the twin spacecraft had their names changed from Mariner 11 and 12 to Voyager 1 and 2. This was done both to reflect their expanded designs and goals beyond what the earlier Mariners had accomplished and to make the probes and their missions more exciting to the public. The Voyager team also hoped that, though the craft were still officially meant to explore just Jupiter and Saturn, they would be strong and adaptable enough to complete most of the original Grand Tour plan by reaching Uranus and Neptune just over one decade hence.

Finally, just months before the two Voyagers would leave Cape Canaveral in Florida aboard separate powerful Titan 3E/Centaur rockets, a small group of far-seeing individuals convinced NASA to place a sampling of sights and sounds of our world and our species engraved onto two golden records which were subsequently attached to the sides of the Voyagers. These discs would accompany the probes past the outer worlds into the wider realm of the Milky Way galaxy. These artifacts would serve as a long-lasting record and tribute to the beings who built and launched these early interstellar wanderers and as a greeting for either their distant children or other intelligences that may move among the stars.

Exploring the Gas Giants

With their missions spanning the second decade of the Space Age, the two Voyagers truly revolutionized our understanding of the outer Solar System, in spite of the fact that they were not the first vessels from Earth to explore that region of our celestial neighborhood. That honor went to Pioneer 10 and 11, which flew past Jupiter in 1973 and 1974, respectively, with Pioneer 11 going on to flyby Saturn in 1979. The Pioneer probes then headed off into interstellar space carrying golden plaques engraved with basic information about humans, our Solar System, and our place in the galaxy. Nevertheless, the improved technologies aboard the Voyagers allowed scientists to surpass what was seen and found at and about those enormous globes by either the Pioneers or Earth-bound astronomers of the era.

At their first destination, Jupiter, the Voyagers revealed the incredibly complex patterns of the planet’s cloud patterns, including the Great Red Spot, which was confirmed to be a hurricane system three times the size of Earth that has been churning in the Jovian atmosphere for at least four centuries. Amazing as this was, what captured even more attention from the scientists, media, and public alike were the four large Galilean moons that circled Jupiter, collectively named after the Italian astronomer who discovered them in 1610. These moons were truly worlds in their own right and not the relatively sedate places initially thought to be.

Image: This dramatic view of Jupiter’s Great Red Spot and its surroundings was obtained by Voyager 1 on Feb. 25, 1979, when the spacecraft was 9.2 million kilometers from Jupiter. Cloud details as small as 160 kilometers across can be seen here. Credit: NASA Planetary Photojournal.

The innermost of the Galilean moons, named Io, turned out to have highly active volcanoes spewing molten sulfur all over its surface and far into space. Alien volcanoes had been seen before, on the planet Mars, but Io’s were anything but extinct, to say nothing of being almost completely unexpected before the Voyagers came on the scene in 1979. The next moon nearest to Io, called Europa, was a contrast: The moon’s surface was icy and smooth, populated by long dark lines across its face, with only a few impact craters large enough to be visible to Voyagers’ cameras. But underneath Europa’s covering of ice appeared to be a different story: A global ocean of briny liquid water perhaps sixty miles deep with twice the volume of all the water on Earth! Though certainly not visible to the instruments of its mechanical discoverers, serious speculations on the possibility for living creatures and what forms they might take in the distant waters of Europa wasted little time in appearing.

Thanks to the Voyagers, worlds that were once hardly even considered as abodes of geological activity and life were now seen as even better prospects for living organisms than the traditional worlds in those categories. Voyagers’ discoveries at Jupiter, perhaps more than any other place the probes would fly past on their journeys out of the Solar System, truly changed humanity’s perspectives on the alien realms inhabiting the outer reaches of our celestial neighborhood. Witnessing the truly dynamic nature of our Solar System through the Voyagers also enriched and expanded our thinking about worlds and beings around other suns, made all the more plausible by the discoveries of extrasolar planets in the years since the primary Voyager missions, of which most thus far appear to be similar to Jovian worlds.

Introducing Ourselves to the Cosmos

Thirty-three years after leaving Earth and twenty-one years after Voyager 2 had flown past the last of the gas giant planets, Neptune, both Voyagers continue to function and return priceless data on regions of the outer Solar System where no human-made spacecraft has ever been before. This area, known as the heliosphere, is considered part of the cosmic boundary between our Solar System and where true interstellar space lies. Perhaps before they expire around 2025, one or maybe both of the Voyagers will last long enough to perform one more scientific mark by revealing the constituents of deep space beyond the influence of our Sun.

The very fact that the Voyagers would be propelled into the Milky Way galaxy by their interactions with the giant planets of the outer Solar System is what inspired the late Cornell astronomer and science popularizer Carl Sagan and others to create what has become known as the Voyager Interstellar Records. While he and others knew the odds of the Voyagers ever being found by other intelligences were small, the fact that the probes would be only our third and fourth artifacts sent to the stars compelled Sagan and his companions to utilize this opportunity to preserve something of ourselves where it could last far longer than anywhere on Earth, perhaps one billion years or more.

Most importantly, while the Voyagers were built and launched by the United States of America, the golden records were designed to represent as much of our whole human species and the rest of life on Earth as possible in images, words, sounds, and music. Scanning through the contents of the records (which can be done from this Web site: http://goldenrecord.org/) one gets a definite sense of our being one species on just one world among hundreds of billions of stars in a vast Universe composed of billions of galaxies. To quote Carl Sagan in the Epilogue of the 1978 book on the Voyager Interstellar Record titled Murmurs of Earth: “But one thing would be clear about us: No one sends a message on such a journey, to other worlds and beings, without a positive passion for the future. For all the possible vagaries of the message, they could be sure that we were a species endowed with hope and perseverance, at least a little intelligence, substantial generosity and a palpable zest to make contact with the cosmos.”

Carrying on Voyager’s Work

Faces from Earth is paying tribute to the Voyager missions and the grand concepts embodied by them and all those who made it possible. For in a very real sense the space probes became reality by a collective effort of our civilization, both directly and indirectly. The Voyagers gave us our first real taste, both through their journeys past immense and amazing alien worlds as never witnessed before and the messages and information they carry in those small golden discs on their sides, of what the Cosmos is really like and how we in turn appear in relation to all that vastness of space and time.

Faces from Earth is keeping alive those representations and messages for the current and future generations of humanity, for we are even now becoming Citizens of the Galaxy, no longer confined to the savannahs or villages or even this single planet. While we are still very much the biological creatures of our distant ancestors, we now have an awareness and abilities they never dreamed of. We must mature into the species we are moving towards, one which embodies and embraces new worlds and new ways of life scarcely imagined in the past or even now. Faces from Earth will work together with all those who share the dream of being part of a humanity achieving its full potential among the stars.

One thing I didn’t realise until recently was the discovery of gravitational slingshots by Michael Minovitch is what made the Grand tours possible. Prior to his work everyone “knew” that the Outer Planets needed nuclear powered rockets to reach them, even on Hohmann trajectories.

Bravo! Larry always engages my interest. Thanks for the update. I love to dream about aliens studying those records.

I want to wax lyrical, forgive me if I phase change here. We should keep in mind that when we are discovered our technical prowess will be viewed in the context of a species imprinted with instincts for survival among primitives. Instincts that are wholly inconsistent with the success of technological civilization. Religion, Bhopal, the Rape of Nanking…oh gosh do i propose to list examples? indeed the list is astronomical…

Our psychological baggage marks us as a natural experiment transpiring here in this pale blue petrie dish. And those golden records are a tick in a very important box. Its not just about E=mcc, the science and hardware. Rather, they say clearly, to any observer, “For at least some of the time, some of us are able to beat the beast in us into submission. Know that we glimpse what can be, what must be.”

No where on the records does it say that we are mad keen residivists. But somehow I suspect that won’t be a secret for long. I hope they can meet us more than half way. And if we are so fortunate, perhaps one day we find another candle in the darkness. Flame on!

The Voyagers were truly groundbreaking. Aside from all of the data they supplied, I find it amazing, and encouraging, that they still retain some functionality.

The Voyagers showed dramatically that we can navigate craft with all the precision necessary for just about any purpose over the breadth of the solar system, and that we can build systems that can continue to function for decades in harsh environments ranging from near Jupiter space to deep space. As amazing and useful as the data they collected is, I think these two accomplishments are the most significant of the Voyagers’ legacy.

Yes the Voyager Interstellar Record serves as much for a snapshot portrait of our species and our world for us as for an ETI . Here is Carl Sagan talking about the Voyager mission ten years after his Cosmos series:

Thank you Procyan. As for showing alien intelligences our “baggage” as well as our accomplishments, the Voyager record team decided not to do this for their message both to avoid confusion and the potential for making an unintended threat or offense. For example, they did not show an image of a nuclear mushroom cloud as it could have been perceived as a primitive yet still possible threat. But who knows how an alien mind might interpret anything on the records.

Perhaps one thing we can be certain of about any creatures in the Universe is that unless they started off as gods they too had their growing pains even if they are near deities now. So at least they may recognize that humanity has a long way to go.

That is part of the mission and the challenge of Faces from Earth, to properly represent our species and our world to the galaxy at large.

This is a nit-pick, but: from the outer planets, the Sun is not “just a very bright star”.

From 30 AU away, the Sun is about one arcminute across. A person with average vision can resolve a disc about half an arcminute across, if it’s light against a dark background. So from the outer planets, the Sun would be a tiny, brilliant disc, not a star.

Also: seen from Pluto at aphelion, the Sun would have an apparent magnitude of around -18.2. A full Moon is about -12.9 So the Sun on Pluto would be about 150 times brighter than a full Moon on Earth.

There’d be no mistaking it for “just another star”.

This is one of those errors that just gets endlessly repeated. No offense.

— And, you know: The Sun seen from the outer planets wouldn’t be like a star, or like the Moon, or like the Sun as we know it. It wouldn’t be like anything in our experience, actually. Isn’t that more interesting than “just another star”?

Doug M.: that actually raises the question of just how dangerous the Sun would be in terms of eye damage at the distance of the outer planets. We did not evolve to deal with such a small but very bright light source, so I wonder if it would manage to trigger the reflex that prevents us from accidentally looking directly into the Sun. Similar issues may arise if we ever manage to get to the Alpha Centauri system. The second star would be a similar small but very bright source.

Doug M. thank you for clarifying just how bright the Sun would look from the outer worlds. I admit I was being a bit poetic to get across just how much more distant Jupiter and the rest are from the inner terrestrial worlds.

On another note, a while back I wrote about how Carl Sagan stated that the Voyager cameras were just powerful enough to image any potential floaters in the Jovian atmosphere:

The floaters were his concept, along with E. E. Salpeter for what one of several types of organisms might exist on a world like Jupiter.

I asked then and I ask again: Has anyone ever scanned the images of Jupiter taken by the two Voyagers for macrolife in the planet’s atmosphere? I know Galileo and Cassini also scanned the planet as well. I think it might be a fun exercise and who knows what one might find in this endeavor.

The Voyagers have already collected so many milestones in their journey through our Sol system and beyond, helping to find extraterrestrial life would essentially be the crowning par for the course for them.

I want to add to the information database about these probes one of my favorite books, the Voyager Neptune Travel Guide, published just months before Voyager 2 flew by the planet Neptune. Very readable and lots of detail on everything about the whole mission.

“This book blooms with such glorious rushes of exalted prose that I was
dog-earing almost every page until I gave up. Contrasting the mission
with human explorations from earlier eras, for example, Pyne writes
that Voyager was “a modernist machine loosed onto the cosmos. The
Voyagers would not be blinded by gold or the mirage of fame. They
would not abandon wife or child, or enslave unwary indigenes. They
could not despair, could not be crippled by loneliness, could not
fight for the cross or suffer for science, would not know epiphanies
or endure tropical fevers. They would lay no claims, issue no
proclamations of sovereignty, raise no toasts to king or republic,
sign no treaties of trade or military alliance, nor send out
reconnaissance parties to lay out routes for folk migration. . . . The
Voyagers confronted no Other, or even life.”

While people frequently refer to space exploration, few think about what exactly it means to explore space. Jeff Foust reviews a book that uses one of the most successful robotic spaceflight missions in history to examine space exploration in a historical context.

I’ve got a treasure for you readers. We take for granted now the ability to get detailed mission updates in a timely fashion via the Web. Many space missions have websites with regular — weekly or monthly or so — science and engineering updates giving the status of spacecraft and their science missions, with information on both the good and the bad, the exciting and the mundane. (Check out, for example, the rovers, Cassini, Deep Impact, and IKAROS.) How did people get their mission status before the Web? Through printed newsletters, of course. And it’s a sad fact that these documents are mostly not available online; in fact, I doubt they’re available in many libraries.

So it’s wonderful that there are space enthusiast pack rats out there who saved this stuff for decades. And it’s even more wonderful that these pack rats have scanners and free time. One such space fan, Tom Faber, sent me an email out of the blue a few weeks ago saying, “I have a set of the Voyager Mission Status Bulletins that JPL published from 1977-1990. I have scanned these and made PDFs of them. I’ve looked for these on the net and haven’t found them posted anywhere so I would like to make them available. Can you suggest who might put these on line?” Can I suggest anybody? How about me?!!

Imagine yourself aboard a spacecraft pushing into interstellar space. At what point would the Sun cease to be the brightest object in your sky? We’re already looking at missions designed to study the local interstellar medium (LISM), with the goal of reaching anywhere from 300 to 400 AU, a region believed to be undisturbed by the Sun. From that range, the Sun still shows an apparent visual magnitude of -13.7, making it brighter than any other star we see from Earth (Sirius comes in at magnitude -1.46).

So it’s a long push. In fact, an early interstellar probe moving at 75 kilometers per second would have to travel six thousand years to reach the point where the Sun is no longer the brightest star. At 100,000 AU, which is 1.61 light years, our imaginary probe occupant would finally see a sky where the Sun was just another bright star.

The information comes from a Mike Gruntman paper that’s referenced in the article.

Emily Lakdawala has linked to a historical NASA book on the Voyager 1 and 2 missions to the first two Jovian systems on The Planetary Society blog, which is excerpted here:

A while ago I posted all 99 issues of the Voyager Mission Status Bulletins in PDF format, and now I have another cool item to add to that collection: NASA EP-191, “The Voyager Flights to Jupiter and Saturn.”

This was a 60-page booklet on nice, thick paper that contained dozens of beautiful, high-resolution, full-color pictures from Voyager 1 and 2’s flybys of the two giant planets and their moons.

More than 30 years after they left Earth, NASA’s twin Voyager probes are now at the edge of the solar system. Not only that, they’re still working. And with each passing day they are beaming back a message that, to scientists, is both unsettling and thrilling.

The message is, “Expect the unexpected.”

“It’s uncanny,” says Ed Stone of the California Institute of Technology in Pasadena, Voyager Project Scientist since 1972. “Voyager 1 and 2 have a knack for making discoveries.”

Today, April 28, 2011, NASA held a live briefing to reflect on what the Voyager mission has accomplished–and to preview what lies ahead as the probes prepare to enter the realm of interstellar space in our Milky Way galaxy.

With the Voyagers now speeding through interstellar space, NASA will host a panel on 28 April to discuss the project’s scientific and philosophical impact. The discussion will stream live at 1:00 p.m. eastern time on both the NASA and Jet Propulsion Laboratory sites.

Voyager’s extraterrestrial invite was spearheaded by the late astrophysicist Carl Sagan. Nick Sagan, Carl’s son, was only 6 when he recorded ”Hello, from the children of Planet Earth” on the disc.

Today he’s an award-winning science fiction writer, best known for his work on TV’s ”Star Trek: Voyager” and the Idlewild book trilogy. His graphic novel series, Shrapnel, is in development for a film and video game, and he has a deal for a show on the Science Channel.

Here, Sagan—who spoke to Spectrum Radio last year—talks about how Voyager inspired him.

Explanation: It is a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter’s Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious. A better understanding of the weather on Jupiter may help contribute to the better understanding of weather here on Earth.

The above image is a recently completed digital enhancement of an image of Jupiter taken in 1979 by the Voyager 1 spacecraft as it zoomed by the Solar System’s largest planet. At about 117 AU from Earth, Voyager 1 is currently the most distant human made object in the universe and expected to leave the entire solar heliosheath any time now.

Thankfully, the spaceship created by conceptual collective New Catalogue and installed at Scottsdale Museum of Contemporary Art isn’t going into space anytime soon.

In 1977, NASA approached American astronomer, astrophysicist, author, and popular science communicator Carl Sagan with a mission: Find out what it means to be a human on Earth and we’ll send it into space, so that intelligent life might one day understand. Sagan and a team of artists compiled a time capsule of 116 images, an assortment of natural sounds and musical collections, and messages spoken in 55 languages on gold-plated audio-visual discs that were attached to NASA’s Voyager 1 and Voyager 2.

Thirty-four years later, New Catalogue (which includes artists Luke Batten, Jonathan Sadler, Mary Vorhees Meehan, and Neil Donnelly along with composer Judd Greenstein) brought its own sounds, images, and a massive “spaceship” installation to SMoCA to revisit Sagan’s mission on a large, interactive scale.

“You are about to enter a spaceship,” reads the exhibition statement by curator Claire Carter.

“There will be many clues in your surroundings — the design language, the constellation of stars, the greetings inscribed on the walls . . . Join them on a journey of exploration — your presence, imagination, even your physical movement, will become part of the artwork.”

The writer Frank White, whose essays on the subject of cosmic scale should be canonical, refers to a shift in perspective called the “Overview Effect.”

White’s estimation, supported by accounts from those in the unique position of having seen the Earth from space, is that such an overview has a penetrating, complex effect. It triggers a singular insight: sudden awareness of life’s interconnectedness and the frailty of our planet.

For those of us on the ground, gazing up into space can be a mutable experience. To some, it’s a horror of the Lovecraftian variety: a deep abyss, out of which some undefinable and eldritch ancientness threateningly emanates. To others, the blackness of space represents a kind of anattā, direct evidence of the non-self.

While the former escape to light-polluted urban centers and live their lives in denial of the vast beyond, the latter meditate under the stars. And yet all of us, no matter our impulses, are at least dimly aware of the significance of our planetary position: we hang suspended in an incomprehensible void.

In my interview with Frank White, he pointed out:

“I find it somewhat puzzling that when we talk about problems on Earth, such as the so-called ‘population problem,’ we never include the dimension of our larger environment, i.e., the solar system and beyond. And when we talk about the ‘energy problem,’ only a few people are willing to even consider the promise of satellites that could beam solar energy to the Earth. We discuss almost every major human problem as if we were confined to one planet, rather than being on ‘Spaceship Earth,’ which is a part of the solar system, galaxy, and universe.”

VOYAGER 1 has been beaming data to Earth since 1977. But members of mission control at NASA’s Jet Propulsion Laboratory (JPL), in Pasadena, California, are as excited as ever. Some time before 2015 the probe should report that it has entered the heliopause, an area where the sun’s “solar wind” is no longer strong enough to beat back the stellar winds of neighbouring stars. At that point its “triaxial fluxgate magnetometer” will detect a change in the direction of the magnetic field perpendicular to its path from east-west to north-south. Voyager 1 would then, the American space agency’s press office insists, become the first manmade object to leave the solar system.

Cynics—as well as most astronomers, cosmologists and, indeed NASA itself—point out that the edge of the solar system is more properly defined as the point beyond which an object will not succumb to the sun’s gravity. Gravity is, after all, what defines the universe at the grandest scale. That, though, is roughly 50,000 times farther from the sun than Earth is. Voyager 1 has so far travelled 123 times Earth’s distance from the sun, or 18 billion km. It would need another 14,000 years, give or take, to escape the sun’s gravitational pull at its current speed.

None of which should detract from the Voyager programme’s momentous achievements. Voyager 1 and its twin, Voyager 2, dispatched 15 days earlier but now trailing because of its excursion to Uranus and Neptune, have offered glimpses of the four gas giants and a slew of strange astronomical phenomena. And while Voyager 1 will remain in the solar system for some time to come, it will soon enter the region where the solar wind’s charged particles give way to a thinner medium of dust and other matter that fill space between stars. Scientists keenly await news from that uncharted territory.

Over the years the Voyagers have sprung a number of astronomical surprises. The latest came last summer, when Voyager 1 encountered a previously unknown phenomenon now dubbed the “magnetic highway”. In this region, its instruments suggest, solar and interstellar magnetic fields meet. Edward Stone, the Voyager programme’s boss since its inception in 1972, explains that this causes them to align in such a way that lower-energy particles from within the “heliosphere” of the sun’s magnetic influence seep out, replaced by higher-energy particles from the expanses of space.

Voyager 1 should also soon be able to settle once and for all that as the sun sweeps through the interstellar medium it does not leave behind a wake. The existence of this “bow shock” used to be astronomical received wisdom but evidence collected in the last few years by the Voyagers and the Interstellar Boundary Explorer (IBEX) craft orbiting Earth indicates that the sun must be travelling too slowly relative to the interstellar medium to produce this effect.

To get where they are, both Voyagers first passed through the “termination shock” (Voyager 1 did so in December 2004; Voyager 2 followed suit in August 2007), a point at which the sun’s solar wind abruptly slows due to pressure from interstellar gases, and entered the heliosheath. The heliosheath was previously thought to be turbulent, yet the Voyagers have measured no solar wind at all, suggesting that solar and interstellar gales cancel each other out. From there, Voyager 1 has gone on to cruise the magnetic highway, with Voyager 2 expected to do so in the coming years.

The hope was that the probes would be sturdy and resilient enough to withstand the predicted trip across the termination shock and buffeting in the heliosheath, as well as other cosmic vagaries on their journeys, which took both perilously close to Jupiter and Saturn, with Voyager 2 making a detour past Uranus and Neptune. So when in 1973 Pioneer 10, launched a year earlier, measured radiation around Jupiter to be substantially higher than anticipated, Dr Stone’s team spent nine months replacing and even redesigning many of the components of the Voyagers, already well in the works in the early 1970s. Of course, the probes were engineered with plenty of redundancy. For example, each carries two copies of three separate computer systems. So far, though, few of the back-up systems ever needed to be deployed. Dr Stone, a glint of paternal pride in his eye, boasts that both vehicles have far exceeded expectations.

Cleverness back on Earth, too, has played a role in the mission’s success. When Voyager 2’s primary and secondary receivers failed a year into its mission, the ground team worked around it to communicate with the back-up system, which remains in use to this day. In 2010, on receiving a garbled message from the probe, the team painstakingly retrieved a full dump of memory using one of the back-ups, which revealed a single bit in a program sequence had flipped from 0 to 1. Reloading the program did the trick.

Nor is the tinkering confined to fixing glitches. The boffins regularly updated control systems to ensure optimal use of the probes’ resources during their most active phases. They did so 18 times during the Jupiter stage of Voyager 1’s mission alone. Take data transfer. When Voyager 1 and Voyager 2 flew around Jupiter and Saturn, the probes were close enough to Earth to send uncompressed images and other data at relatively high bit rates of 115,000 and 45,000 bits per second (bps), respectively. Because signal strength varies as the inverse square of the distance between receiver and transmitter, however, by the time it reached Uranus, Voyager 2 would be transmitting at 9,000bps. This would drop to 3,000bps around Neptune, substantially reducing the number of images and readings that could be beamed home.

Most back-up computers are designed to operate only if the primary system fails. However, one of the probes’ auxilliary systems was hooked up so it and its primary could run in parallel when needed. Before Voyager 2 flew by Uranus, simple image-compression software was transmitted to the back-up, allowing a 640 kilobyte picture to be squeezed without loss to as little as 256 kilobytes.

Most ingeniously of all, Dr Stone’s team equipped the probes with an advanced bit of hardware called a Reed-Solomon encoder. The device significantly reduced the burden of “error-correction” necessary to ensure that a message can still be read if some of the bits are lost or corrupted during transmission. Originally, the Voyagers used an older, well-tested system that sent one “error-correcting” bit for every bit of the actual message; error correction therefore hogged half the available bandwidth. The Reed-Solomon encoder would enable a single bit to correct for five bits of the message. The rub was that in 1977 a way to decode Reed-Solomon corrected data had yet to be worked out. Luckily, by the time Voyager 2 reached Uranus in 1986, it had been.

At present the data trickle in from the Voyagers to the Deep Space Network of radio telescopes sprinkled around the globe (which capture information from all manner of space and planetary operations), and thence to JPL, at just 160bps. This was a conscious decision made years ago to maintain a constant rate for the remainder of the mission. The crafts’ cameras were turned off after planetary fly-bys, and just a few instruments remain active. For about 30 minutes every six months data stored during that period on an 8-track digital tape are transferred at a zippier 1,400 bps.

The Voyagers’ radioisotope thermoelectric generators, ticking away with the decay of their plutonium-238 fuel, should keep the instruments and transponders going until at least 2021. Then, around 2025, after nearly half a century of boldly going where no manmade object had gone before, the team will shut down the probes while communications are still available in a somewhat sentimental effort to keep the Voyagers aimed straight and true. And they will drift deeper into interstellar darkness.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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